Decoupling Transcription from Cell Fate Preserves Germ Cell Totipotency.

Only germ cells can give rise to a totipotent embryo, yet a conserved transcriptional program for germ cell identity remains elusive. Profiling Drosophila primordial germ cells (PGCs) across embryogenesis, we find that nascent PGCs are not silent, as previously thought, but instead transcribe the same totipotency program as the early embryo, reflecting an open chromatin state. This chromatin state, established by the pioneer factor Zelda, persists in PGCs throughout development. The associated transcriptional program normally drives somatic differentiation, but in PGCs the conserved RNA-regulator Nanos represses its translation, leaving germline chromatin uncommitted. Cells that fail to establish the totipotent state die before reaching the gonad. Thus, germ cells preserve developmental potential not by restricting transcription, but by preventing transcripts from determining cell fate.